Astrophysicists have determined with unprecedented accuracy that our Sun is located halfway between the Sagittarius Arm (closest to the galaxy center) and the Perseus Arm of the Milky Way Galaxy. The distance from the Sun to each of the spiral arms is 6400 light years. (Image Credit: NASA/JPL/R. Hurt - CalTech)

Astrophysicists have found that the Perseus spiral arm of the Milky Way Galaxy lies only half as far from Earth as previous studies had suggested. An international team of astronomers measured a highly accurate distance to the Perseus arm for the first time using a globe-spanning system of radio dishes known as the Very Long Baseline Array (VLBA), which offers the sharpest vision of any telescope in existence.

"We know less about the structure of our own galaxy than we do about many nearby galaxies like Andromeda," said astronomer and team leader Mark Reid of the Harvard-Smithsonian Center for Astrophysics. "We literally can't see the forest for the trees because we are embedded inside our own galaxy, and interstellar dust blocks our view."

Previous estimates of the distance to the Perseus arm varied by a factor of two. Studies based on the motions of stars yielded a distance of more than 14,000 light-years, while observations comparing the apparent brightness of massive, young stars with estimates of their intrinsic brightness yielded a distance of only about 7,200 light-years. The new VLBA measurements confirm with an accuracy of 2 percent that the Perseus spiral arm is located about 6,400 light-years from the Earth.

Obtaining accurate distances in astronomy is a difficult challenge. The most reliable method for measuring astronomical distances is called trigonometric parallax, a technique similar to the triangulation used by land surveyors. A trigonometric parallax is determined by observing the change in position of a star relative to a very distant, essentially fixed object like a quasar, as the Earth moves in its orbit around the Sun. Just as a finger held at arm's length appears to shift against the far wall when viewed with one eye or the other, a nearby object will appear to shift position relative to a more distant one. Mathematical calculations then yield the distance to the closer object. The parallax method is powerful but requires exceptional accuracy.

"I have spent more than a decade developing the calibration techniques we needed to obtain this result," said Reid.

The team achieved an accuracy of 10 micro-arcseconds, which is a factor of 100 better than previous methods. That resolution is equivalent to looking from the Earth to a person standing on the Moon's surface and telling whether that person is holding a flashlight in their right or left hand. The VLBA is the only telescope able to provide such high resolution.

With these successful results in hand, the team has been awarded additional VLBA observing time to measure other regions of the Milky Way. Over time, these studies will help map the spiral structure of our galaxy and determine the distribution of unseen dark matter believed to surround it.

The VLBA is part of the National Radio Astronomy Observatory (NRAO), a research facility of the National Science Foundation (NSF). Dedicated in 1993, the VLBA consists of 10, 25-meter-diameter dish antennas spread from Hawaii to St. Croix in the Caribbean. The antennas all work together as a single telescopic system roughly the size of the Earth.